1. Field of the Invention
This invention relates to a method and apparatus for the measurement of a physical property of a fluid that is dependent upon a physical characteristic of at least one functional group and that is related to the quantity of that functional group in the fluid. In one aspect, this invention relates to the measurement of the heating value of a fuel gas at-line and in real-time. In one aspect, this invention relates to a method of self-calibration for an apparatus for measuring the physical property of interest of a fluid, such as the heating value of a combustible gaseous fuel mixture. In one aspect, this invention relates to a method and apparatus for measuring the heating value of a combustible gaseous fuel mixture, including functional groups and molecules, using near-infrared absorption spectroscopy.
2. Description of Related Art
Historically, the heat energy content of a combustible fluid was determined by burning precisely defined amounts of the fluid, such as natural gas, to determine the amount of energy produced from the combustion. Other methods determined concentration of each whole combustible compound in the mixture, defining the energy content for each whole combustible compound, and summing them to yield the heat energy content of the entire mixture.
The heat energy content of natural gas flowing through a pipeline, which natural gas typically contains methane, ethane, propane, and higher alkane hydrocarbons, frequently fluctuates, even over relatively short periods of time. Conventional methods of measurement generally require bypass flow-lines or fluid extraction to provide gas samples which are then taken to a lab and burned. The temperature of the flame is then measured. Available sensors for making these measurements are primarily comprised of calorimeters and gas chromatographs. Disadvantageously, such devices, in addition to requiring the removal of samples from pipelines, have slow response times, and have high initial and maintenance costs. It is difficult to both continuously and accurately measure the energy content of natural gas in pipelines, and the lack of any convenient means for making such continuous and accurate measurements may result in improper charges during the course of a day to the disadvantage of both buyers and sellers.
One method and apparatus for addressing the need for both continuous and accurate measurement of the heat energy content of combustible gaseous fluid mixtures is described in U.S. Pat. No. 7,248,357, which is incorporated herein in its entirety by reference. As described therein, a method and system is provided for measuring the heat energy of a combustible fluid in which radiation means direct radiation through a sample of the combustible fluid, detection means detect absorbance of at least one combustible components of the combustible fluid at a selected spectral line, where there is at least one spectral line for each combustible component to be considered in the combustible fluid, calibration means calibrate the source of the radiation, storage means store a plurality of spectra of combustible gas mixtures, thereby enabling comparison of the measured absorbance spectrum to the plurality of spectra, combination means combine at least one heat energy proportional factor with the absorbance at each spectral line, and summing means sum the combinations to determine the heat energy of the combustible fluid. The system continuously acquires absorption spectra from gases in the near-infrared region. The near-infrared region of the electromagnetic spectrum is particularly useful because combustible gas components, in particular methane, ethane, propane, butane, iso-butane, and hexane produce strong absorbent spectra in this spectral range. The measurement of absorption values at several predetermined wavelengths allows reconstruction of fuel composition and heating value using specially developed mathematical algorithms. The absorbance value is calculated as
  A  =      ln    ⁡          [                        I          0                I            ]      where I0 is the light intensity measured with an optical cell filled by purging gas and I is the intensity of light measured with the cell filled with a fuel. Calibration (zeroing) of the system requires periodic flushing of the optical cell with a purging gas, such as nitrogen or air. The disadvantages of this calibration method include system complexity due to the requirements for a purging gas supply and additional valves and controls and interruption in sensor monitoring when purging is taking place.
FIG. 1 is a schematic diagram of a conventional spectroscopic heating value sensor. As shown therein, the sensor comprises optical cell 10 having optical windows 11, 12 and input and output gas connectors 13 and 14. Periodic switching between fuel and purging gas flows is performed by valve 20. A stabilized light source 21 produces a light beam 22 that is passed through the cavity 23 of the optical cell. The light exiting the optical cell through optical window 12 is dispersed by spectroscopic instrument 24 and directed to a near-infrared sensor array 25 measuring absorption at various wavelengths. The resulting signal is amplified by amplifier 26 and provided to data processor 27 for processing.
FIG. 2 shows the characteristic variations of pressure, P, fuel concentration, F, and light absorption, I, at a given wavelength with time for a gaseous fuel sample. As shown therein, the pressure is constant during the process. The fuel/purging gas flow is altered with valve 20, resulting in a periodic change of fuel concentration with time. The variation in fuel concentration results in changes of absorption signal I from its maximum value I0 corresponding to purged cell conditions to the level obtained with the optical cell completely filled by fuel, IF. The absorption value is calculated as
  A  =      ln    ⁡          (                        I          0                          I          F                    )      
The characteristic time of the cycle tc is limited by the requirement of flushing the cell completely and is typically about five minutes, during a substantial portion of which sensor monitoring of the sample gas cannot be conducted.